Electrical devices in tightly closed casings, such as for example an engine control unit, are freely exposed to environmental influences. In active state, these devices heat the air inside the casing. This expands and leads to a positive pressure inside the casing. When the electrical devices are then switched off, they cool down and the interior pressure falls. In principle, the exterior temperature can change substantially more quickly than in a well-sealed casing, for example in sudden, severe summer thunderstorms. In air-tight electrical casings, such temperature fluctuations firstly lead to condensation and secondly the seals are stressed by the increased and reduced pressure, so that the seal protection is weakened over time. A further problem is that on a reduced pressure inside the casing, additional moisture can be drawn into the inside. Since the moisture cannot easily escape again, it can precipitate on the casing when temperature differences occur between the casing interior and the environment, and in some cases collect on the base. On repeated temperature fluctuations, the water quantity can increase over time and damage the installation, leading to corrosion and short-circuits. Depending on application, failure of the installed components and devices may cause serious damage.
Pressure compensation elements serve to ventilate electrical devices in tightly closed casings such as are used for example in motor vehicle construction. Such pressure compensation elements largely prevent the penetration of water into the casing interior, but allow a compensation for example of pressure differences between the internal air and the environment due to temperature changes. From the prior art, pressure compensation elements are known wherein a gas-permeable membrane is glued or welded directly onto a bore present on the casing of the electrical device. To protect the membrane, in addition a covering label or cap is glued or stuck directly onto the membrane. It has proved disadvantageous that the membrane can lose its permeability due to various solid body particles, such as for example dust and dirt, whereby faults and failures of the electrical devices can occur. Replacing the membrane is either totally impossible or complicated, cost-intensive and time-consuming.
Furthermore, pressure compensation elements are known which have base bodies with a membrane welded or glued directly onto the base body. Such pressure compensation elements are applied to the casing with separate sealing elements. In addition to the above-mentioned problem of the welded or glued membrane, it has proved problematical that, for example, during installation sealing elements can be forgotten and/or the entire pressure compensation element installed incorrectly. Installation of the pressure compensation element or replacement of the membrane is cost-intensive and a source of possible installation errors, whereby again faults and failures of the electrical devices can result.
WO 2008/077667 A2 proposes a pressure compensation element which has a pot-like cover element with a base surface and a wall, and an adapter inserted in the cover element which has a continuous bore. The opening of the bore is covered by a membrane which is attached to the top end face of the adapter. An air passage is formed between the inner wall of the cover element and the outer wall of the adapter, and between the base surface of the cover element and the bore of the adapter. The membrane is supported by webs applied in a cross pattern in the bore to prevent the membrane from folding.